In normal photographic film, an emulsion containing silver halide covers a transparent substrate. The silver halide in the emulsion is sensitized in those areas that are exposed to actinic radiation. The degree of sensitizing of every area of the emulsion is a function of the intensity and duration of the radiation striking that area. The latent image of sensitized silver halide that is formed by the exposure to actinic radiation is "developed" by immersing the emulsion in a chemical "developer" which converts the silver halide to microscopic grains of very black metallic silver.
After the developing operation, the developer is washed out of the emulsion, and the emulsion is "fixed" by immersing the emulsion in a chemical which, among other effects, dissolves the unconverted silver halide out of the emulsion. This leaves behind the image in the form of grains of black metallic silver in a density that is the inverse or reverse of the image impressed on the emulsion. That is, in areas where high levels of actinic radiation struck the emulsion, a very large proportion of the silver halide was sensitized and converted to black metallic silver. The high density of black metallic silver grains results in an area of the developed image which is very dense and dark. In areas where less actinic radiation strikes the emulsion, a much lower proportion of the silver halide is converted to black metallic silver; and the portion of the resulting image in those areas is very light with almost no granules of black metallic silver present in the developed emulsion.
The normal development process described above results in a "negative" image of the object photographed, that is, the image tonality is opposite of the object to which the emulsion was exposed. Such emulsions that are normally developed to produce a negative image are said to be "negative acting." If a positive image of the object is desired, that is, an image where tonality is the same as the object to which the emulsion was exposed, this may be obtained by exposing through the negative image to another photographic emulsion. This two-step method for obtaining a positive image is the one most commonly used. There are, however, other methods which will produce a positive image in one step. Two methods of particular importance are "reversal processing" and "positive acting" duplicating emulsions.
In reversal processing, the emulsion is exposed as usual, and the initial developing step is completed to develop the sensitized silver halide to black metallic silver. After the developer has been washed out of the emulsion, the emulsion is not fixed but rather is subjected to a bleaching bath, of a type well-known to those skilled in photography. Such bleaching baths customarily include dilute sulfuric acid and potassium dichromate, which oxidizes and thus removes the black metallic silver negative image from the emulsion without affecting the remaining undeveloped silver halide. Therefore, after the bleaching operation, the emulsion is again clear and carries a latent "positive" image of the object originally photographed, in the form of unexposed and undeveloped silver halide.
After the bleaching operation, the emulsion is immersed in a clearing bath which neutralizes any remaining portion of the bleaching chemicals remaining in the emulsion. The emulsion is then subjected to a general exposure to actinic radiation which sensitizes all of the remaining silver halide. This reexposed image is then developed in a developer bath to convert all of the remaining silver halide to black metallic silver. After development, the emulsion is washed and then fixed to remove any spurious silver halide which has not been properly operated on by the radiation and developer. The resulting image is a positive transparency of the object photographed.
In order to obviate the extra steps involved in reversal processing of normal emulsions, "duplicating film" is often used in the production of positive images for use in the graphic arts industry. Duplicating film uses one of many well-known image-reversal effects in which the film is first exposed overall to actinic radiation or to a chemical fogging treatment. This exposure "fogs" the film by sensitizing the silver halide grains over the entire emulsion. If the film were then to be developed, it would be a uniform black.
In order to employ one of those effects, the fogged, undeveloped film is exposed to actinic radiation by exposing the film to an illuminated object such as a line drawing or a page of typed copy.
The exposure to the actinic radiation from the light areas of the object desensitizes the silver halide of the emulsion. Thus, actinic radiation lightens the latent image in the emulsion in those areas where the actinic radiation from the object strikes the emulsion. Normal development of this type of film results in a high density of black metallic silver in those areas which were initially fogged and exposed to the dark areas of the object. It also results in a lesser density of the developed image in the emulsion in those areas which were exposed to lighter portions of the object being photographed.
In addition to the many and well-known normal-processing, image-reversal phenomena such as the solarization, Clayden, Villard and Herschel effects, a number of patents have issued in recent years directed to shell-grain or covered grain, positive acting emulsions for normal-processing, positive image photography. Reference is made to U.S. Pat. Nos. 3,206,313 granted to Porter et al. on Sept. 14, 1965; 3,632,340 granted to Illingsworth on Jan. 4, 1972; 3,367,778 granted to Berriman on Feb. 6, 1968 and 3,317,322 granted to Porter et al. on May 2, 1967.
Manufacturers of positive acting film do not normally publish information disclosing the manner in which their emulsions achieve a positive image. However, a widely-used, positive acting or positive image duplicating film is marketed by the Eastman Kodak Company under the designation LPD precision line film. E. I. DuPont de Nemours & Company (Inc.) Photo Products Department, Wilmington, Del. 19898, also markets a positive acting duplicating film under the designation CRONALAR SD. These films are exposable in white light and exhibit a photographic "speed" comparable to high-contrast, orthochromatic negative-working reproduction films such as Eastmen Kodak KODALINE Reproduction Film 2566.
Photographic image definition is often used to express the overall quality of the image and is sometimes broken down into many different aspects including: tone, graininess, sharpness and resolution. While tone is important in some applications, the preferred embodiment of the present invention is directed more to high contrast uses of photographic processes in which tone is not a factor. Graininess is controlled in the manufacture of the emulsion and the developers used and is not particularly affected by the present invention. The present invention improves image definition by affecting the sharpness and resolution of the image in a photographic emulsion.
It is well known in the photographic art that reversal processing to produce a positive image reduces the definition of the film and makes for a fuzzier and less precise image on the emulsion.